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Fall on an outstretched hand with the forearm in excessive pronation (hyper-pronation injury). The Ulna fractures in the proximal one-third of the shaft due to extreme dislocation. Depending on the impact and forces applied in each direction, degree of energy absorption determines pattern, involvement of the radial head and whether or not open soft tissue occurs.

Direct blow on back of upper forearm would be a very uncommon cause. In this context, isolated ulnar shaft fractures are most commonly seen in defence against blunt trauma (e.g. nightstick injury). Such an isolated ulnar shaft fracture is not a Monteggia fracture.[citation needed] It is called a 'nightstick fracture'.

Osteosynthesis (open reduction and internal fixation) of the ulnar shaft is considered the standard of care in adults. It promotes stability of the radial head dislocation and allows very early mobilisation to prevent stiffness. The elbow joint is particularly susceptible to loss of motion.

In children, the results of early treatment are always good, typically normal or nearly so. If diagnosis is delayed, reconstructive surgery is needed and complications are much more common and results poorer. In adults, the healing is slower and results usually not as good.

Complications of ORIF surgery for Monteggia fractures can include non-union, malunion, nerve palsy and damage, muscle damage, arthritis, tendonitis, infection, stiffness and loss of range of motion, compartment syndrome, audible popping or snapping, deformity, and chronic pain associated with surgical hardware such as pins, screws, and plates. Several surgeries may be needed to correct this type of fracture as it is almost always a very complex fracture that requires a skilled orthopedic surgeon, usually a 'specialist', familiar with this type of injury.[5]